mpitraits.hh forward declares std::pair, which is illegal by the standard, and...

mpitraits.hh forward declares std::pair, which is illegal by the standard, and clang stumbles over that.
Fixes FS#1121.
(Thanks to Steffen Müting for the patch)

[[Imported from SVN: r6791]]

COPYING

@@ -19,7 +19,7 @@ Copyright holders:
 2010--2011    Andreas Lauser
 2007--2011    Sven Marnach
 2010          Rene Milk
-2011          Steffen Müthing
+2011--2012    Steffen Müthing
 2003--2006    Thimo Neubauer
 2011          Rebecca Neumann
 2008--2012    Martin Nolte

dune/common/mpitraits.hh

@@ -8,6 +8,8 @@
 #include "mpi.h"
 #endif
 
+#include <utility>
+
 namespace Dune
 {
   /** @addtogroup ParallelCommunication
@@ -143,10 +145,6 @@ namespace Dune
     }
   };
 }
-namespace std
-{
-  template<typename T1,typename T2> struct pair;
-}
 
 namespace Dune
 {